Casing or liner drilling advances a casing or liner string at the same time as the bit on the drill string makes more hole. One significant concern when doing casing or liner drilling is the close clearance around the casing or liner that is defined by the open hole and the risk of sticking the casing or the liner string before reaching the desired depth. Additionally the annulus equivalent circulating density (ECD) is significantly higher than in standard drill pipe drilling situations. Drilling with Casing exerts a higher pressure on the wellbore and may require lower circulation rates or risk losing the wellbore prematurely.
An additional benefit of increasing the annulus between the casing and the wellbore is to allow more cement in place for additional protection and increased security for a complete cement bond.
FIG. 1 illustrates a known system for casing drilling. A drilling rig 5 drives the casing string 12 that has a drill bit 14 at a lower end 16. FIG. 1 is not drawn to scale. The annulus 26 between the casing string 12 and the borehole wall 22 is small and can lead to stuck pipe and higher ECD's.
FIG. 2 illustrates a liner drilling application where a drill string 30 supports a liner 32 through a liner hanger 39 and has a bottom hole assembly that comprises of a drill bit 34 and a bottom hole assembly (BHA) 31. The BHA 31 comprises, for example, of the drill bit 34 a steerable device 35 for deflecting the well bore, an MWD system 37 and an underreamer 36.
Typically the underreamer 36 does not increase the borehole wall sufficiently to increase the clearance for the liner. In the prior art a special coring bit 40 or another stationary reamer further increase the hole to the new borehole wall 44. Due to the fixed width of the core bit or the stationary reamer 40 the annulus is slim and leads to the already mentioned operational problems. This clearance is given by the drift internal diameter of the previous casing string and operators typically accept this deficiency for the benefit of Liner Drilling.
The underreamer 36 when fully extended provides a minimal clearance from the outer surface 42 of the liner 32 and the borehole wall 44. A stationary reaming device 40 is mounted to the outer surface 42 to somewhat increase the clearance for the liner 32 created by the underreamer 36. The clearance increase from the reaming device 40 is marginal over the clearance that would have been there without reaming device 40. There are limits to the blade extension of underreamer 36. Trying to ream bigger hole sizes 44 with reaming tool 36 weakens the blades of the underreamer 36 with a risk of bending or fatigue breaking them creating the potential risk that the underreamer 36 will not be able to collapse for extraction through the liner 32 or a risk that parts could be lost in the hole.
The present invention focuses on an articulated reamer mounted to the casing or the liner so that the reaming starts from the outer surface and can better assure that a clearance is provided to the open hole so that the casing or liner will not stick even when negotiating a well deviation. Details of some ways to accomplish the reamer extension and the retention of the reamer blades or components are described. Those skilled in the art will understand from the description of the preferred embodiment and the associated drawings additional details of the present invention while understanding that the full scope of the invention is to be found in the appended claims.